This invention relates to a process for bonding films to one another or to other substrates by means of a two-component adhesive containing an isocyanate component and a hydroxyl component which adhesive is cross-linkable by electron beams or UV-light.
In practice, lamination (i.e. bonding) of films to form composite films is generally carried out by means of adhesive components dissolved in organic solvents. The function of the solvent is to convert the highly viscous, partly solid adhesives into a form in which they can be applied. In order to achieve satisfactory bonding, however, the solvent must be removed from the adhesive system after application to one of the films and before the films are bonded to form a composite film.
This time-consuming process step may be avoided by using solvent-free laminating adhesives. In one known solvent-free adhesive, highly viscous urethanes containing terminal isocyanate groups are brought to the appropriate consistency by heating to temperatures around 100.degree. C. This adhesive, upon application to the film, hardens under the effect of moisture to form a bond (cf. for example German Offenlegungsschrift No. 2,549,227). Problems encountered in using such systems include inadequate stability of these reactive adhesives in the heated applicator (pan life) and weak initial adhesion in the composite film. In addition, these known systems generally have the same disadvantages as moisture-hardening one-component adhesives, i.e., the hardening rate depends upon the quantity of moisture present in the environment which of course varies greatly according to the type of film and the time of year. To overcome this disadvantage, moisture often has to be artificially introduced into the bonding gap in the form of finely atomized water.
Ethylenically unsaturated prepolymers such as those obtained by reacting hydroxy alkyl (meth)acrylates, polyisocyanates and, optionally, polyether polyols and/or polyester polyols, have been described as solvent-free radiation-hardening adhesives which can be hardened by UV-light or electron beams. Adhesive compositions of this type are described, for example, in French Pat. No. 2,361,452, U.S. Pat. No. 4,018,333 and in German Offenlegungsschrifts Nos. 2,324,822, 2,323,373 and 2,102,382. Although these ethylenically unsaturated prepolymers may produce good results as adhesive components in some applications, they have not been used successfully in laminations where layer thicknesses are typically 0.5 to 5 .mu.m (1 .mu.m=10.sup.-3 mm) at most.
German Offenlegungsschrift No. 2,913,676 describes a process for the production of composite films by means of solvent-free adhesives in which oligomeric and/or polymeric esters and/or ethers containing both free isocyanate groups and free (meth)acrylate groups in one molecule are used as liquid adhesives. These systems have the same disadvantages as one-component urethane adhesive systems in which the bond reaches its maximum strength after about 1 to 3 weeks.
Solvent-free two-component systems made up of a polyol component and a (poly)isocyanate component are also known (see, for example "Kaschieren ohne Losungsmittel (Lamination without Solvents)", Papier und Kunststoff-verarbeiter, No. 7 (1978)). Systems of this type generally show significantly greater final adhesion than one-component systems. In addition, they can be modified to have specific desired properties by varying the formulations. This adaptability makes these two-component systems useful for a large number of applications.
In all of the known solvent-free one-component and two-component systems, however, the composite film generally shows only minimal initial adhesion immediately after lamination. For example, when the composite film is wound into a roll, this poor initial adhesion may result in longitudinal shifting of the films (telescope effect) or in the formation of voids and wrinkles (orange-peel effect).